And some things came immediately to mind. Wearing flannel everywhere, for every occasion. Although that might be more of a "growing up in the Northwest" thing, and not a 90s thing. JNCO jeans. But those seem to be more fashion based. Here are some of the answers that popped up on the AskReddit thread.

Pilots allowing kids into the cockpit to have a look around and be f***ing amazed.

Having a question and not immediately looking up the answer.

Playgrounds with all metal everything and gravel bases.

Walking all the way to the departure gate at the airport with your family/friends even if you weren't traveling with them.

Prior to call ID, calling someone multiple times without getting a response was pretty normal. So was looking someone up in the phone book.

Smoking indoors.

Having a main character in a sitcom act grossed out by homosexual public displays of affection.